Repeatable Read Isolation in SQL running via background worker

I am using worker_spi as a model to run a SQL statement inside a background
worker. From my browsing of the Postgres library, I believe that if I want
repeatable read isolation level, the proper way for me to attain this is to
add this line after StartTransactionCommand() in worker_spi_main:

XactIsoLevel = XACT_REPEATABLE_READ;

Or - am I mistaken? Does PushActiveSnapshot already ensure I will get the
same snapshot of the data within this transaction?

Can anyone help me if this is accurate or if there are any other gotchas I
should be aware of?

The SQL statement will be run every minute for example, and each time with
this isolation level. At least, that is my goal.

Any help is much appreciated.

Thanks,
Jeremy